1. Field of the Invention
The present invention relates to a technique for implementing a current source in field effect transistor technology.
2. Description of the Prior Art
Most linear circuits are biased by means of a current source. It is usually thought desirable that this source provide a current that is independent of temperature, power supply, and process variations. One current source in common use takes advantage of the logarithmic insensitivity of a bipolar transistor's forward base-emitter voltage, VBE, to power supply and process variations. A resistor placed across the emitter-base junction of an active transistor (FIG. 1) will give a reference current equal to VBE/R. CMOS (Complementary Metal-Oxide-Semiconductor) integrated circuits have also used this technique by taking advantage of the intrinsic bipolar transistor in the CMOS structure. Unfortunately, this current source has a large temperature dependence, since VBE has an intrinsic negative temperature coefficient of approximately -2 mv/degree C., and the resistor has a positive temperature coefficient. Hence, the current from this source has a large negative temperature coefficient.
A great deal of work has been done on circuits that provide a constant reference voltage, but relatively less on the apparently similar job of producing a constant reference current. In the case of field effect transistor (FET) current sources, steps are frequently taken to mitigate the effects of large lot-to-lot variations in device parameters, for which field effect transistors are notorious. In particular, circuits are usually designed to minimize the effects of threshold and gain variations that occur for field effect transistors on different wafers. For example, a resistor is typically included in the source path of a FET to provide degenerative feedback, which reduces these variations.